I found it.
for future reference
function HerbArmorMimage::onMount(%this, %obj, %slot, %client)
{
%client = %obj.client;
if($Pref::Avatar::Chest == 1)
{
%client.player.unmountimage(2);
%client.player.mountimage(HerbArmorFimage, 2);
}
}
It could probably be more complex, because it does not change if you change your chest, but who realy does that?